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Issue No. 14, Article 10/June 24, 2005

Rust Is Common on Many Crops, but Not Yet on Soybeans in Illinois

Recently, we have had conversations with people who refer to "rust" and ask what is happening with this disease. We, appropriately, then ask what kind of rust they are referring to. Some say "soybean rust," of course, since that is the hot news item; others express surprise that there are other kinds of rust diseases on plants. Most, if not all, of the audience that reads this article will know about common rust of corn and rusts on wheat and perhaps other kinds of plants. But we felt there was an interest among some people about learning more about rust diseases and rust fungi, since they are among the most common and damaging diseases of crops and other plants worldwide.

In Illinois, stripe rust of wheat caused concern in some fields this spring (see the Bulletin article in issue no. 10, May 27, 2005). Last year, common rust of corn was a concern in some cornfields in Illinois. In addition, as noted in Table 2, rust diseases affect a wide range of agricultural and horticultural plants, including oats, turf grass, alfalfa, white pine and apple trees, asparagus, coffee, sunflower, raspberries, sunflowers, and dry beans. Each of these rusts is distinctly different from both of the soybean rusts, and most of them are fairly host-specific, meaning there is generally no cross-infection between them.

Rust refers generally to a type of fungus that infects aerial parts of plants, primarily leaves and stems. Rust fungi are typically spread by the wind, often have alternate hosts to complete their life cycles, and develop characteristic pustules that cause breaks in the surfaces of infected plants. Spores (urediniospores) develop in the pustules and are blown to other plants, where new infections may occur if weather conditions are favorable for disease development.

This article is intended to familiarize you with rust diseases that may occur in Illinois on a variety of horticultural and agricultural plants. Some of the rusts addressed are established in Illinois and quite common; others are rare, not established, or of regulatory concern. The occurrence of each rust disease in Table 2 represents the collective experience of the table authors: Nancy Pataky (director, University of Illinois Plant Clinic), Dr. Mohammad Babadoost (Extension specialist, University of Illinois), and Dr. Ian Thompson (curator, Purdue University Arthur [Rust] Herbarium). Rather than providing detailed symptom and biology descriptions for each rust disease mentioned, we have listed links to specific Internet resources.

To many, "the rusts" are among the most interesting and complicated plant diseases. The following points summarize the main sources of intrigue and confusion:

  • Nearly all rust fungi are obligate parasites, which means they cannot be grown apart from a living host. Thus, conducting research with rust pathogens is relatively difficult.
  • Rust fungi may produce as many as five different stages in their life cycles:
    • Stage 0: Spermogonia-bearing spermatia (sexual recombination occurs here)
    • Stage I: Aecia-bearing aeciospores
    • Stage II: Uredinia-bearing uredin-iospores (summer or repeating spores)
    • Stage III: Telia-bearing teliospores
    • Stage IV: Basidia-bearing basidio-spores
  • For some rust fungi, all five stages of the life cycle have been observed (macrocyclic); however, other rust fungi, including soybean rust, lack one or more stages and may be referred to as microcyclic (which produce only teliospores and basidiospores), demicyclic (which do not produce urediniospores), or endocyclic (which only produce spermatia and aeciospores).
  • Some rust fungi are autoecious, meaning they survive and complete their life cycle on a single host species.
  • Some rust fungi (such as cedar-apple rust) are heteroecious, meaning they can complete their life cycle and survive only by alternating between two very different host species. In these cases, the signs and symptoms on the two plant species can be radically different, causing observers to wrongly conclude that there are two distinctly different pathogens at work.
  • Some heteroecious rust fungi (such as common rust of corn and wheat leaf rust) appear in Illinois each year, even though the alternate host(s) is absent or rarely infected in our region. These rusts don't overwinter in our northern climate but arrive as windblown (or via infected plants) urediniospores from southern regions of the United States.
  • Soybean rust is also expected to fit the pattern of survival in the southern United States and movement into Illinois via windblown spores, and then production of urediniospores as a repeating stage on infected plants.
  • In terms of managing rusts, it is important to know how and when the pathogen arrives, and whether it produces a repeating/summer spore (urediniospore) stage.

For a more detailed explanation of the rust fungi, the technical terms used, and their history, consider visiting the following Web sites:

--Bruce Paulsrud and Dean Malvick

Authors:
Dean Malvick
Bruce Paulsrud

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